Process for the preparation of polyurethanes containing uretdione rings formed in the presence of a bismuth salt of an organic carboxylic acid

ABSTRACT

Polyurethanes containing uretdione rings are produced by reacting a polyisocyanate containing a uretdione ring with a relatively high molecular weight dihydroxyl compound in the presence of a bismuth salt of an organic carboxylic acid and a low molecular weight aromatic diamine. The low molecular weight aromatic diamine is used in a quantity which is 5 to 10 times the quantity of bismuth salt used. These solid polyurethanes are formed in relatively short reaction times and possess outstanding mechanical properties.

BACKGROUND OF THE INVENTION

This invention relates to a process for the preparation of polyurethanesfrom diisocyanates containing uretdione rings.

It is known in the art (DE-PS No. 1,153,900) to react diisocyanatescontaining uretdione groups with relatively high molecular weightpolyhydroxyl compounds and optionally difunctional chain lengtheningagents to produce polyurethanes. In order to achieve sufficientpolyaddition of the uretdione-containing diisocyanates (which aregenerally high melting and only sparingly soluble), it is usuallynecessary to employ reaction temperatures above 100° C. Use of suchtemperatures may however result in the opening of the uretdione ringsand the initially straight chained polyurethanes may undergo a muchgreater degree of cross-linking than desired.

Substantial lowering of the solidification temperature and shortening ofthe solidification time in the curing process by using catalysts of thekind conventionally used in polyurethane chemistry, e.g. organic metalsalts or tertiary amines have been attempted. However, the density ofcross-linking is found to be undesirably increased and the mechanicalproperties of the polyurethane are seriously impaired.

The same problems are encountered when solid uretdione diisocyanateswith delayed reactivity such as those which have been described in DE-OSNo. 3,230,757 are used for the preparation of storage stable mixtures ofso-called one component systems for the production of polyurethanes. Inthese cases, again, the use of organic lead or tin compounds as thepreferred catalysts cannot completely prevent opening of the uretdionerings and the attendant undesirable side reactions.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to optimize the knownprocesses for the preparation of polyurethanes from the usually highmelting and sparingly soluble diisocyanates containing uretdione ringsso that back formation into diisocyanates and opening of the uretdionerings with formation of biuret and allophanate groups and hencecross-linking would to a large extent be avoided. It is also an objectof the present invention to provide a process for the production ofpolyurethanes in which the polyaddition products rapidly change fromtheir wax-like state to the desired elastic end state and could bereleased from the mold after a very short time for further processing.

These and other objects which will be apparent to those skilled in theart are accomplished by reacting an organic polyisocyanate which is atleast 50 wt. % diisocyanate containing a uretdione ring with arelatively high molecular weight dihydroxyl compound in the presence ofa catalyst and a low molecular weight aromatic diamine. The catalyst isa bismuth salt of an organic carboxylic acid. The aromatic diamine isused in a quantity which is from 5 to 10 times the quantity of bismuthsalt used.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for the production ofoptionally foamed polyurethanes by the reaction of organicpolyisocyanates containing at least 50 wt. % of a diisocyanate havinguretdione rings with a relatively high molecular weight dihydroxylcompound preferably with a molecular weight of from 500 to 10,000, andoptionally low molecular weight diols as chain lengthening agents,preferably with a molecular weight of from 62 to 400, in the presence ofa catalyst, a low molecular weight aromatic diamine and optionally otherauxiliary agents and additives. The catalyst used is a bimuth salt of anorganic carboxylic acid. The low molecular weight aromatic diaminepreferably has a molecular weight of from 108 to 600 and is used in aquantity of from 5 to 10 times the quantity of bismuth salt.

The use of bismuth salts as catalysts for the preparation ofpolyurethanes from conventional diisocyanates has already been describedin U.S. Pat. No. 4,584,362 but polyisocyanates containing uretdionerings were not mentioned. When polyisocyanates containing uretdionegroups or their delayed action forms (which are preferably used inone-shot processes for preparing polyurethanes from storage stablesystems) are reacted with hydroxyl compounds in the presence of bismuthsalts, the desired rapid reaction and short solidification time areobtained. However, faults such as cavities or so-called "shell cracks"are found in the end products if the reactive system is exposed toatmospheric moisture and the mechanical properties of the products areseriously impaired. It has now surprisingly been found that when organicbismuth salts are used in combination with low molecular weight aromaticdiamines, the desired short reaction times with high conversion ratesand the desired solidification times are obtained and substantiallyperfect end products with outstanding mechanical properties areproduced.

The bismuth salts of organic carboxylic acids used are preferably saltsof organic carboxylic acids having from 2 to 20 carbon atoms.Bi(III)-2-ethylhexanoate has been found to be a highly effective bismuthactivator. Particularly suitable organic bismuth salts are thoseprepared from organic carboxylic acids corresponding to formula (I)

    R--COOH                                                    (I)

in which R represents a C₁ -C₂₀ -alkyl, C₄ -C₂₀ -cycloalkyl, C₆ -C₂₀ orC₇ -C₂₀ aralkyl group and inorganic bismuth salts. The R group ispreferably methyl, octyl, cyclohexyl, phenyl, tolyl or naphthyl.

The organic bismuth salts are preferably used in a quantity of 0.01 to 5wt. %, more preferably 0.05 to 2.0 wt. %, based on the quantity of thestarting components.

In the process of the present invention, the bismuth activators are usedin combination with low molecular weight aromatic diamines. The aromaticdiamines are added to the starting materials in 5 to 10 times thequantity of bismuth salt. It is immaterial whether the aromatic diaminesare used in the solid or liquid form. If they are solid, however, theparticle size should not exceed 50 to 100μ in a one-component system.

Examples of low molecular weight aromatic diamines, includinghetero-aromatic amines, which preferably have molecular weights in therange from 108 to 600 include p-phenylene diamine;2,4/2,6-tolylene-diamine; diphenylmethane-4,4'- and/or 2,4'- and/or-2,2'-diamines; 3,3'-dichloro-4,4'-diaminodiphenyl-methanes; 3-(C₁ -C₈-alkyl)-4,4'-diaminodiphenyl-methanes; 3,3'-di-(C₁-C₄)-4,4'-diaminodiphenyl-methanes; 3,3',5,5'-tetra-(C₁-C₄)-alkyl)-4,4'-diphenylmethanes; 4,4'-diaminodiphenyl-sulphides,-sulphoxides and -sulphones; diamines containing ether groups such asthose disclosed in DE-A No. 1,770,525 and 1,809,172 (U.S. Pat. Nos.3,654,364 and 3,736,295); 2-halogen-1,3-phenylene-diamines optionallysubstituted in the 5-position (DE-A Nos. 2,001,772, 2,025,896 and2,065,869); bis-anthranilic acid esters (DE-A Nos. 2,040,644 and2,160,590); 2,4-diamino-benzoic acid esters (DE-A No. 2,025,900) andtolylenediamines substituted by one or two (C₁ -C₄)-alkyl groups.Particularly preferred are 3,5-diethyl-2,4- and/or -2,6-diaminotoluene(especially their commercial 80/20 or 65/35 isomeric mixtures),asymmetrically tetraalkyl substituted diaminodiphenyl methanes such as3,5-diethyl-3',5'-diisopropyl-4,4'-diaminodiphenylmethane,4,4'-diaminobenzanilide, 3,5-diaminobenzoic acid-(C₁ -C₄)-alkyl esters,4,4'and/or 2,4'-diaminodiphenylmethane, and naphthylene-1,5-diamine.

The diisocyanates containing uretdione rings (dimeric diisocyanates)used in the present invention are preferably solid diisocyanates withmelting points above 40° C., preferably above 80° C., most preferablyabove 130° C. These have been described in DE-OS No. 3,403,500. Examplesof such diisocyanates include: tolylene-2,4-diisocyanate, dimeric4,4'-diisocyanato-diphenylmethane and its oligo dimers, straight chainedhomologs (e.g. with up to 4 uretdione rings in the molecule), dimeric2,4'-diisocyanato-diphenylsulphide, dimeric4,4'-diisocyanato-diphenylsulphide, dimeric4,4'-diisocyanatodiphenylsulphone and all other known aromatic uretdionediisocyanates and mixtures of such uretdione diisocyanates. Dimerictolylene-2,4-diisocyanate and dimeric 2,4'- and/or4,4'-diisocyanatodiphenylmethane are preferred.

The preparation of uretdione diisocyanates from the correspondingdiisocyanates is well known. (See Hofmann, Berichte 3 (1870), pages765-766.) The preparation of, for example, dimerictolylene-2,4-diisocyanate has been described in detail inKunststoffhandbuch Volume 7, Polyurethane, published by Vieweg/Hochtlen,Carl-Hanser-Verlag, Munich 1966, page 16. More specifically, uretdionediisocyanates may be prepared by dimerization of the above-mentioneddiisocyanates with catalysts such as trialkylphosphites (DE-OS No.2,349,726), peralkylated carbamoylphosphites (U.S. Pat. No. 3,290,288),peralkylated aminophosphines (U.S. Pat. No. 3,290,288), 3- or 4-substituted pyridines such as 4-dimethylaminopyridine (GB-PS Nos.821,158, 944,309, 962,689), trialkylphosphines (highly reactivesubstances sensitive to air, DE-OS No. 2,420,475), dialkylarylphosphinesand alkyldiarylphosphines (U.S. Pat. No. 2,671,082), trialkylarsines(Analytical Chemistry of the Polyurethanes, Volume 16/III, High PolymersSeries (Wiley 1969), pages 112 to 131), dibutyl tin dilaurate (DE-OS No.2,420,475), without a catalyst in a carboxylic acid ester (USSR-PS No.149,775) or in aqueous emulsion (GB-PS No. 1,134,285).

Oligomeric uretdione diisocyanates corresponding to the general formula(II) ##STR1## in which X represents an aromatic group and

m represents an integer of from 1 to 5,

may also be used in the practice of the present invention. The uretdionediisocyanate obtained from 4,4'-diphenylmethane diisocyanate (preferred)may advantageously be prepared at low temperatures (e.g., 20° C.) by theaddition of a catalyst in only slightly polar solvents such as aliphatichydrocarbons (m=1-2).

In addition to the diisocyanates containing uretdione rings, up to 50wt. %, based on total isocyanates, of other diisocyanates may be used.Examples of such isocyanates are given in DE-OS No. 3,230,757.

If the preparation of polyurethanes by the process of the presentinvention is carried out with a so-called one-component PU reactivesystem, then the diisocyanates are preferably used in blocked form, i.e.with retarded reactivity. The preparation of diisocyanates with retardedreactivity is known in the art and has been described, for example, inDE-OS No. 3,230,757 and DE-OS No. 3,403,500.

The other starting compounds used for the preparation of polyurethanesin accordance with the present invention include relatively highmolecular weight compounds containing H-active groups of the kindcommonly used in polyurethane chemistry. These compounds preferably havemolecular weights in the range of from 500 to 10,000 and the H-activegroups are mainly hydroxyl groups but may also include amino groups.Specific examples of such compounds are given in DE-OS No. 3,230,757.

Low molecular weight chain lengthening agents which may optionally beused include diols, preferably with molecular weights of from 62 to 400,in which the hydroxyl groups are preferably attached to aliphatic and/orcycloaliphatic groups. These compounds are disclosed, e.g., in DE-OS No.3,230,737.

Other optional auxiliary agents and additives include: dyes andpigments; fillers such as silica gel, gypsum, talc and active charcoal;UV absorbents; stabilizers such as phenolic anti-oxidants; lightprotective agents; blowing agents; surface-active additives such asemulsifiers or foam stabilizers and optionally cell regulators;anti-blocking agents; silicones; flame retardants and fungistatic and/orbacteriostatic substances.

The preparation of polyurethanes according to the present invention maybe carried out by any of the known procedures such as those using aone-component or a two-component system. The use of a one-component PUreactive system as starting mixture is preferred.

Suitable one-component systems may be prepared, for example, by theprocedure described in DE-OS No. 3,230,757. More specifically, a soliddiisocyanate preferably having a particle size of from 5 to 100μ andpreferably in its blocked form is mixed with a relatively high molecularweight H-active compound, optionally low molecular weight diol, theorganic bismuth salt, the low molecular weight aromatic diamine andoptionally other additives. The system is preferably heated to atemperature of 100° C. to 150° C. to produce the polyurethane.Sufficiently solidified polyurethanes or molded polyurethane productswith a faultless structure and excellent mechanical properties can beremoved from the mold within a very short time (i.e., 1 to 2 minutes).

The process employed for shaping the one-component system depends uponits characteristics. A liquid system which is pourable at 25° C. may beworked up by a casting process and the system may be briefly heated(e.g. to 50°-70° C.) before it is processed.

Systems which are not sufficiently pourable to be cast but are to someextent fluid may be applied to their supports by means of a coatingknife and then solidified by a thermal shock.

Plastic systems (pastes) may be molded under heat and pressure.

Solid systems, especially those based on diols with a relatively highmelting point (45° to 65° C.) may either be molded under pressure(injection molded) or shaped at or above the melting point of the diol.A previously prepared one-component system may be converted into a solidgranulate and introduced in this form into a mold which has been heatedto a temperature above the melting point of the diol (generally below70° C.). When the granulate has melted and thus filled the mold, themold is heated to 100°-120° C. and the contents are solidified.

The elastomeric polyurethanes produced by the process of the presentinvention are suitable for use as foils or sheets, as fabric coatings,e.g. on textiles and/or conveyor belts and for the production ofcastings.

The invention is further illustrated but is not intended to be limitedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLES Example 1

1000 g of a linear polyester of adipic acid and diethylene glycol (OHnumber 43) which was liquid at 25° C. were thoroughly dehydrated at 100°to 120° C. in a vacuum. 1.5 g of1-amino-3,3,5-trimethyl-5-aminomethylcyclohexane (IPDA) were then addedat room temperature. After the further addition of 170 g of dimeric2,4-diisocyanatotoluene (TT) in the form of a finely ground powder(particle size 5 to 20μ ), the uretdione diisocyanate was evenlydistributed in the reaction mixture by means of a stirrer. Thesuspension had a viscosity of about 25,000 mPas/RT and was stable instorage at room temperature for several months. The product graduallysolidified but only in the heat and only inferior products wereobtained. In order to achieve satisfactory solidification of the samplesat lower temperatures, the catalysts shown in Table 1 were added to thevarious samples of reaction mixtures and one set of samples was workedup as described below in the absence of atmospheric moisture whileanother set was worked up after 4 hours' exposure to atmosphericmoisture.

The solidification of samples 2 to 3 mm in thickness after exposure tothe given temperature for 2 minutes was determined on an electrichotplate adjusted to 120° C. The ease with which the plates (˜5×5 cm)could be lifted from the tack-free surface was used as thecharacterizing feature.

                                      TABLE 1                                     __________________________________________________________________________                      Exposure to Heat                                                                       Atmospheric moisture                                                                      Property of the                        Catalyst   Quantity (g)                                                                         (min)    with  without                                                                             polyurethane                           __________________________________________________________________________    Bismuth octoate                                                                          0.2             tack-free                                                                           tack-free                                                                           homogeneous                            Diamine.sup.1                                                                            1.0    2        surface,                                                                            surface,                                                                            structure                                                         rubber-like                                                                         rubber-like                                  Bismuth octoate                                                                          0.2    2        tack-free                                                                           tacky non-                                                              surface,                                                                            surface                                                                             uniform                                                           rubber-like structure                              Diazo-bicyclooctoate.sup.2                                                               0.5    2        tack-free                                                                           tacky non-                                                              surface,                                                                            surface                                                                             uniform                                                           rubber-like structure                              Lead octoate                                                                             0.2    2        tack-free                                                                           tacky non-                                                              surface,                                                                            surface                                                                             uniform                                                           rubber-like structure                              __________________________________________________________________________     .sup.1 Diamine = 2,4diamino-3,5-diethyltoluene-                               .sup.2 Available under the name Dabco from Air Products                  

Sample plates were then prepared from the resulting polyurethanes by theusual methods. After they had been tempered at 120° C. for 2 hours, theelastomers showed marked differences in their mechanical properties.Those mechanical properties are reported in Table 2.

                                      TABLE 2                                     __________________________________________________________________________              Degasified without atmospheric moisture                                       Bismuth   Diazo     With atmospheric                                          octoate/                                                                           Bismuth                                                                            bicyclo-                                                                           Lead moisture                                        Catalyst  diamine.sup.1                                                                      octoate                                                                            octoate.sup.2                                                                      octoate                                                                            Bismuth octoate/diamine                         __________________________________________________________________________    Shore hardness A                                                                        82   80   60   62   80                                              DIN 53 505                                                                    Tensile strength                                                                          22.5                                                                               19.5                                                                               2.2                                                                                2.1                                                                                20.5                                          (MPa) DIN 53 504                                                              Tear propagation                                                                        800  750  150  100  800                                             resistance (KN(m))                                                            DIN 53 515                                                                    Elasticity (%)                                                                          47   45   35   36   46                                              __________________________________________________________________________

The mechanical properties of elastomers prepared with lead octoate anddiazo-bicyclooctoate were so poor that they were unsuitable for use ascoating materials even when prepared with the exclusion of atmosphericmoisture. Products prepared with bismuth octoate had improved mechanicalproperties and a uniform structure only if they were prepared with theexclusion of atmospheric moisture. Products with excellent mechanicalproperties and a uniform structure even when prepared in the presence ofatmospheric moisture were obtained only with the bismuth salt anddiamine combination according to the invention.

Example 2

150 g of liquid reaction mixtures prepared according to Example 1 with(a) 0.15 g of Bi-octoate or (b) 0.15 g of Bi-octoate and 3 g of2,4-diamino-3,5-diethyltoluene were applied to metal sheets (zinc platedsheet iron) to form films about 1 mm in thickness. The film coatedsheets were then exposed to atmospheric moisture (85% relative humidity)for about one hour. The films were then solidified at 120° C. Thefollowing results were obtained:

    ______________________________________                                        Catalyst       Results after solidification                                   ______________________________________                                        Bismuth octoate                                                                              Severe blistering, shell cracks                                Bismuth octoate/                                                                             Perfect, homogeneous film                                      2,4-diamino-3,5-                                                              diethyltoluene                                                                ______________________________________                                    

Example 3

500 g of a linear polypropylene glycol ether (MW 2000, OH number 56)were mixed with 151 g of dimeric TDI (TT) in the form of a milled powderand the suspension obtained was thoroughly homogenized by means of amechanical stirrer. 20 g of 2,4-diamino-3,5-diethyltoluene were thenadded. The mixture obtained was stable for several hours at roomtemperature and the viscosity rose only slowly. When the mixture washeated (120° to 150° C.), however, cross-linking was unsatisfactory andthe material was in part non-uniform and could be destroyed undertension and pressure. It was only when 2.0 g of a 50% bismuth octoatesolution (bismuth 2-ethylhexanoate in cleaning petrol) were added thatsatisfactory solidification to an elastic polyurethane took place underthe action of heat.

Sample plates were produced by storing the reaction mixtures at roomtemperature in the presence of atmospheric moisture for 2 hours and thencasting them into a standard mold which had been treated with moldrelease agent and then heating them in the mold at 120° C. for 30minutes to one hour.

The mechanical properties of the elastomer were determined after a shortstorage time at room temperature.

    ______________________________________                                                         Bismuth octoate/2,4-diamino-                                 Catalyst         3,5-diethyltoluene                                           ______________________________________                                        Hardness (Shore A)                                                                             90                                                           Modulus (100%) (MPa)                                                                           7.5                                                          Tensile strength (MPa)                                                                         11.5                                                         Elongation at break (%)                                                                        450                                                          Tear Propagation resistance                                                                    30.5                                                         (KN/m)                                                                        Elasticity (%)   54                                                           ______________________________________                                    

Example 4

The 500 g of polypropylene glycol ether used in Example 3 were replacedby 500 g of a linear polyoxytetramethylenediol having a molecular weightof 2000 (OH number 56). This diol was combined with 127.8 g of TT, 1.0 gof bismuth ocotate and 10 g of the diamine used in Example 3. Highlyelastic polyurethanes having the mechanical properties shown below wereobtained after 2 hours storage in atmospheric moisture andsolidification by heat.

    ______________________________________                                                         Bismuth octoate/2,4-diamino-                                 Catalyst         3,5-diethyltoluene                                           ______________________________________                                        Hardness (Shore A)                                                                             88                                                           Modulus (100%) (MPa)                                                                           5.0                                                          Tensile strength (MPa)                                                                         10.3                                                         Elongation at break (%)                                                                        500                                                          Tear propagation resistance                                                                    25.6                                                         (KN/m)                                                                        Elasticity (%)   65                                                           ______________________________________                                    

Example 5

1.0 g of bismuth octoate was added to a suspension of 650 g of a lineardihydroxy polybutadiene having a molecular weight of 2600, 10 g of2,4-diamino-3,5-diethyltoluene and 117.3 g of dimeric TDI (TT). Thereaction mixture, which was liquid at room temperature, was solidifiedin the heat after being subjected to the procedure described in Example3 and 2 hours' storage in atmospheric moisture. Highly elastic moldedproducts were again obtained. The mechanical properties are shown below:

    ______________________________________                                                         Bismuth octoate/2,4-diamino-                                 Catalyst         3,5-diethyltoluene                                           ______________________________________                                        Hardness (Shore A)                                                                             84                                                           Modulus (100%) (MPa)                                                                           5.5                                                          Tensile strength (MPa)                                                                         5.5                                                          Elongation at break (%)                                                                        100                                                          Tear propagation resistance                                                                    10.5                                                         (KN/m)                                                                        Elasticity (%)   63                                                           ______________________________________                                    

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A process for the production of a polyurethane inwhich(a) an organic polyisocyanate which is at least 50 wt. %diisocyanate containing a uretdione ringis reacted with (b) a relativelyhigh molecular weight organic dihydroxyl compoundin the presence of (c)a bismuth salt of an organic carboxylic acid and (d) a low molecularweight aromatic diamine in a quantity which is from 5 to 10 times thequantity of bismuth salt (c) used.
 2. The process of claim 1 in whichthe bismuth salt (c) is used in a quantity of from 0.01 to 5 wt. % basedon the total weight of the reactants.
 3. The process of claim 1 in whichthe bismuth salt (c) is used in a quantity of from 0.05 to 2.0 wt. %based on the total weight of the reactants.
 4. The process of claim 1 inwhich the bismuth salt (c) is a salt of an organic carboxylic acidhaving 2 to 20 carbon atoms.
 5. The process of claim 1 in which thebismuth salt (c) is Bi(III)-2-ethyl hexanoate.
 6. The process of claim 1in which the aromatic diamine (d) is 3,5-diethyl-2,4-diaminotoluene,3,5-diethyl-2,6-diaminotoluene or mixtures thereof.